The present invention relates to a method for recycling a substrate, especially a wafer, that has a residue on its surface and a detachment profile resulting from an implantation process. The method includes removing the residue from the substrate to a level substantially equivalent to that of the detachment profile, thus obtaining a substantially uniform planar surface on the substrate, and then polishing the entire surface of the substrate to eliminate defects.
Recycling or reprocessing of substrates, especially semiconductor wafers in semiconductor technology, offers the opportunity to save a lot of money in new material costs. A wafer recycling process involves essentially three main steps: removing unwanted materials, polishing, and cleaning.
Wafer recycling plays a particularly important role in the SMART-CUT® process, shown schematically in FIGS. 1a to 1f. In this example, a first silicon wafer 1 is oxidized to form an oxidized layer 3 and then implanted with gaseous species like hydrogen ions or gas ions 4 (FIG. 1c). The implantation creates a buried, weakened layer comprising microcavities, platelets and/or microbubbles. The implanted wafer is bonded to a second silicon wafer 2 (FIG. 1d) and then detached by splitting at a depth corresponding to the penetration depth 5 of the implanted species (FIG. 1e). The result is the production of a SOI (silicon on insulator) wafer that includes the original second wafer 2 and a SOI layer 7, and a remaining delaminated wafer 1′, which is a smaller portion of the first silicon wafer 1.
The delaminated wafer 1′ cannot be reused directly because it includes a damaged and rough surface 8 which is surrounded by a collar 9 in the form of a step located about the peripheral part of the wafer 1′. The collar 9 is formed during bonding in the SMART-CUT® process wherein the surfaces of both wafers 1 and 2 are bonded except for an edge portion where the wafers are not flat. In particular, when the first wafer is detached from the second wafer, part of the surface of the first wafer 1 which is bonded with the second wafer 2 is transferred onto the second wafer 2. The excluded edge portion which is not bonded is not transferred, and it forms the collar 9 of the delaminated wafer 1′. A loss of material also results at the edge of the positive wafer containing the original wafer 2 and a SOI-layer 7 split from the first wafer 1.
In order to reuse the detached, delaminated wafer 1′, the non-uniform detachment profile shown in FIG. If must be made planar. A chemical and/or mechanical polishing (CMP) is conventionally used to remove the residual material from the uneven topography on the wafer surface to obtain a uniform planarization globally across the wafer. A load force is applied to the entire surface 8 including its collar 9 of the wafer 1′ while it rests on a pad (not shown). The pad and the wafer are then counter-rotated while a slurry containing abrasives and reactive chemicals is passed underneath. This recycling method requires too much material to be removed from the wafer and has a long processing time which is necessary to get a reasonably acceptable value of the total thickness variation (TTV) of the recycled delaminated wafer. This limits the amount of recoverability of the wafer. Furthermore, there is a risk that the wafer may break during planarization, especially when the wafer is repeatedly recycled.
U.S. Pat. No. 6,284,628 describes a method for recycling a delaminated wafer of the above mentioned type in which the delaminated wafer surface is polished to remove about 1 μm material from the surface and is then heat-treated in a reducing atmosphere at a temperature between 1000° C. and the melting point of the wafer material for several hours. Such thermal treatment places a strong load on the treated wafer and requires a lot of energy and processing time.
It would be advantageous if a method were available that does not include thermal treatment, which allows a substrate to be reused more often, and that produces recycled substrate with a low total thickness variance such that a good planar surface quality may be obtained.